1. Field of the Invention
This invention relates to a phase voltage detection circuit applied to an AC power generator for a vehicle, for example, and a power generation control device employing the phase voltage detection circuit.
2. Description of the Related Art
An AC power generator for a vehicle typically includes a power generation control device, a field coil, an armature coil, and a rectifier. The power generation control device supplies an excitation current to the field coil, and controls a power generation voltage by adjusting the excitation current.
At this time, a phase voltage signal generated by the armature coil is input into the power generation control device, whereupon the phase voltage signal is detected by a phase voltage detection circuit and waveform-shaped so as to form a digital signal. The power generation control device executes processing to detect an engine rotation speed and so on using a digital circuit on the basis of the waveform-shaped phase voltage signal.
Normally, an AC power generator for a vehicle supplies an excitation current to the field coil in order to start generating power upon reception of an activation signal from a vehicle control unit or an activation signal linked directly to an ignition key, these activation signals being transmitted over a signal line connected to the vehicle side. Here, when an open fault or a ground short fault occurs on the signal line, the activation signals cannot be received from the vehicle during engine startup, and as a result, power generation cannot be started.
However, power generation can be started even in this condition by making the vehicle AC power generator capable of independent power generation. More specifically, when the vehicle AC power generator rotates, a miniscule phase voltage signal is generated in the armature coil of the vehicle AC power generator by residual magnetic flux even in a case where an excitation current has not been supplied to the field coil by the power generation control device. As a result, a rotation condition of the engine can be estimated from the generated phase voltage signal.
In this case, it is necessary to detect the miniscule phase voltage signal generated by the residual magnetic flux as described above. Moreover, a frequency of the phase voltage signal is commensurate with the engine rotation speed, and therefore the phase voltage signal must be detected within a rotation range used by the engine. Accordingly, the phase voltage detection circuit is required to detect a phase voltage signal that is generated by residual magnetic flux and has an amplitude of no less than approximately 0.2 V and a frequency range of several hundred Hz.
Note that when a leak current is generated in a diode constituting the rectifier prior to power generation, the leak current flows into the phase voltage detection circuit, leading to an increase in a DC bias voltage of the phase voltage signal. Here, the leak current varies in accordance with manufacturing irregularities, contact resistance, and deterioration over time in the diode, and therefore the DC bias voltage of the phase voltage signal also varies. As a result, the phase voltage signal cannot be detected using a fixed threshold voltage.
Hence, to solve the problem described above, a technique of determining an envelope of the phase voltage signal using a peak hold circuit and generating a variable threshold voltage using the obtained envelope so that the phase voltage signal can be detected even when the DC bias voltage of the phase voltage signal is indefinite has been disclosed (see Japanese Patent Application Publication No. 2002-125398, for example).